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Minnesota Stormwater Manual

Guidance and examples for using the MPCA Estimator
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Caution: The MPCA Simple Estimator is being updated. The guidance on the following page is being modified to align with the updated version of the Estimator.
Overview of the MPCA Simple Estimator
The MPCA Simple estimator is an Excel-based tool that utilizes the Simple Method to estimate total suspended solid and/or total phosphorus loads and load reductions associated with implementation of best management practices (BMPs). The spreadsheet includes the following features.
  • 10 worksheets where the user provides inputs to estimate loading. Each worksheet includes the following.
    • Multiple land uses
    • Adjustable event mean concentrations, runoff coefficients, pollutant removal efficiency, and percent of runoff treated by a BMP
    • Adjustments in loading associated with non-structural BMPs, such as street sweeping and changes in land use
  • A worksheet summarizing pollutant loads and reductions across the ten worksheets described above.
  • An information sheet with links to useful pages in the stormwater manual
Caution: The MPCA Simple Estimator contains default values for event means concentrations, runoff coefficients, BMP pollutant removal efficiency, and fraction of runoff treated by BMPs. If appropriate, the user should adjust these values to match site conditions. Guidance for adjusting values is provided on this page.

This User Guide is intended to assist MS4 (Municipal Separate Storm Sewer System) permittees in the completion of the Commissioner-approved TMDL Annual Reporting Form (TMDL Form).

Estimator downloads and links to resources

A quick guide for the Estimator is available Quick Guide: MPCA Estimator tab.

NOTE: This page is a User's Guide that explains the various cells and worksheets in the Estimator. Guidance for using the Estimator for permit compliance and case studies are found at the following links

Opening the MPCA Simple Estimator

When opening Version 3 of the Estimator you will see an information box stating the following. "This preadsheet contains several default values, such as pollutant concentrations, curve numbers, and BMP pollutant removal efficiency. It is important to adjust those values, as appropriate, to your site conditions. Read the information tab for more information."

Click OK to open the spreadsheet.

General description of the MPCA Simple Estimator

The MPCA Estimator spreadsheet presents an optional calculator approach to computing pollutant loading and the pollutant load reduction for total phosphorus (TP) and total suspended solids (TSS)(note: the Estimator may not be used for any other pollutants). Results from the Estimator can be used in the Cumulative reductions tab of the Annual Report form. The Estimator applies load reductions to specific structural stormwater BMPs but can be used to estimate reductions associated with other BMPs. It is a simplistic tool and should not be used for modeling a stormwater system or selecting BMPs.

When working in the Estimator, the following color coding applies.

  • Grey cells have fixed information and cannot be edited
  • White cells can be edited by the user
  • Yellow cells contain default values that the user can edit. Editing the defaults triggers an alert informing the user that the default has been modified.
  • Red cells contain formulas that result in calculated pollutant reductions; these cells cannot be edited

The Estimator utilizes the Simple Method to estimate pollutant loads for total phosphorus and total suspended solids (TSS). BMPs (best management practices) included in the Estimator are biofiltration (bioretention with an underdrain), infiltration (a bmp with no underdrain), filter strip, landscaped roof (green roof), permeable pavement with an underdrain, sand filter, swale, wet basin, and constructed wetland. Users may include other BMPs if they have reliable data on pollutant removal efficiency.

The Estimator allows the user to make calculations for 10 areas (e.g. subwatersheds). Within each area the Estimator computes pollutant reduction using BMP performance data as published in this manual. The Estimator computes the load reduction according to the formula

Load Reduction for a BMP = Load in – Load out = (EMC in * Volume in) – (EMC out * Volume out).

This is then converted to a percentage reduction

% Reduction = Load Reduction ÷ Load in.

The cumulative reduction is the sum of load reduced for all BMPs across all 10 areas (worksheets). This computed reduction can be input into the Cumulative reductions tab of the TMDL Annual Report form.

The Estimator can only be used for one TMDL at a time. If a Permittee has multiple TMDLs and chooses to use the Estimator, separate calculations must be made for each TMDL.

Caution: The results from the MPCA Estimator are intended to show a generalized value that represents approximate progress towards meeting TMDL WLAs. The MPCA Estimator should not be used for other modeling or compliance purposes. Instead, Permittees should use field investigations combined with more detailed models.

Structure of the MPCA Estimator spreadsheet

File:Estimator main image.png
Screen shot of draft Version 3 of MPCA Estimator showing 10 calculation worksheets, an information sheet, and a summary sheet. Click on image to enlarge.

The MPCA Simple Estimator (Estimator) spreadsheet contains 12 worksheets.

  • 1 worksheet providing information about the spreadsheet
  • 10 worksheets where data are entered and calculations made.
  • 1 summary worksheet

Calculation worksheets

Each of the 10 calculation worksheets represents a sub-watershed. The user is not obligated to use 10 worksheets for their study area. We encourage the user to use separate worksheets for the following conditions.

  • The study area has multiple well-defined sub-watersheds.
  • Contributing areas to BMPs in downstream portions of treatment trains should be treated as sub-watersheds and should not include the area contributing to upstream BMPs in the treatment train, particularly if significant amounts of runoff are being treated by the upstream BMPs
  • To more accurately reflect differences in land use. Although the Estimator provides flexibility in selecting land uses, certain situations may be better dealt with by treating land uses as different sub-watersheds. For example, a single residential area highly impacted by emerald ash borers may be treated as a separate sub-watershed from an adjacent residential area where tree canopy will remain intact (i.e. due to significant differences in runoff event mean concentrations).
Information: Each worksheet in the Estimator is called an Area and represents a sub-watershed as discussed above

The 10 calculation worksheets are identical and each contains 5 sections, described below.

  • Calculation of unadjusted total loads for total phosphorus (TP) and total suspended solids (TSS)
  • Calculation of adjusted total loads for TP and TSS
  • Calculation of TP loading reductions associated with implementation of best management practices (BMPs)
  • Calculation of TSS loading reductions associated with implementation of best management practices (BMPs)
  • BMP and land use input values

Each of these sections is described below.

Section 1: Calculation of unadjusted total loads

screen shot of simple estimator
Screen shot of Section 1 of the Estimator. Unadjusted pollutant loads are calculated in this section. Note that defaults values exist in many of the yellow cells, but the user can change these cells. To avoid a calculation error, the default area for each land use is 0.00001 acres, which creates a small amount of pollutant load. Values in red are calculated and cannot be changed by the User. Grey cells cannot be edited. White cells are input cells for the user.
File:Estimator section 1 b.png
Screen shot of Section 1 of the Estimator. In this image, the user has changed some of the default emcs for phosphorus. Note an alert box is shown in those rows where a default has changed. In this example, the user provides a note explaining the emcs are based on monitoring data.
Information: The Estimator spreadsheet uses a mix of SI and English units. Correction factors are included in all calculations. The user must use the correct units for input values.
Caution: To avoid calculation errors, each land use should have a minimum default area when there is acreage within that land use. The Estimator uses a value of 0.00001 and we recommend retaining that value or using a smaller value

In this section, the total unadjusted load, in pounds, is calculated for the area considered. Unadjusted means there is no consideration of reductions associated with practices such as street sweeping, pollution prevention, changes in land use, etc.

This section comprises Cells A5 through L24 of each of the 10 calculation worksheets. In this section, the user inputs land area, in acres, associated with different land uses within the area being considered. The user inputs annual precipitation, in inches. A link provides access to precipitation information if the user does not know the precipitation for the sub-watershed. The section contains default values for TP and TSS event mean concentrations (mg/L) and default values for runoff coefficients. EMCs and runoff coefficients can be changed by the user. Changing a default value triggers an alert box informing the user that the default has been changed.

Column A - Land use

  • Description: This column contains various classifications for land use. Only classifications having adequate information for pollutant concentrations in the literature were included. The User may include additional land uses if adequate information exists for pollutant concentrations in runoff from that land use. The User may also include just a single land use to represent the entire area within the TMDL Project watershed, although this may require estimates of pollutant concentrations and runoff coefficients for areas with multiple land uses.
  • Input: Land use category. Options include commercial, industrial, institutional, multi-use, municipal, open space, residential, park, agriculture, transportation, and water. The User may input up to five other land use classifications.
  • Source of Information: MPCA input or User input.

Columns B and C, Rows 6 through 23 - Event mean concentrations

  • Description: The influent Event Mean Concentrations (EMCs) of the pollutants of concern for each classification of land use listed in Column A.
  • Input: Default value with an option for the user to enter a different value. If the user changes an emc for commercial, industrial, institutional, multi-use, municipal, residential, open space, or transportation, an alert displays in Column I or Column J, informing the user that they have changed the default emc. The user can change emcs for water, park, transportation, or user-specified land uses without an alert.
  • Source of Information: We conducted a literature review to develop information on emcs. We selected default values based on analysis of data from the literature review and using best professional judgement. Information on emcs, including a summary of the literature review and ranges in emcs for different land uses, is found at these links.

Column D, Rows 6 through 23 - Area of specified land use

  • Description: Area of specific land use within the sub-watershed, in acres.
  • Input: Drainage area, in acres. A value must be entered into this cell for the pollutant reduction to be calculated. Cells are therefore populated with a default value of 0.000001 acres to avoid returning an error in the calculations.
  • Source of Information: User. TMDL reports provide maps and acreages for TMDL study areas. Shapefiles of TMDL study areas can be found at [1].

Column E, Rows 6 through 23 - Annual precipitation

  • Description: Total annual precipitation in sub-watershed.

Note: The default value is 30.65 inches per year, which is the average annual precipitation at the Minneapolis-St. Paul International airport. The User should input the appropriate value for their location. The references below can be used to determine this value.

Column F, Rows 6 through 23 - runoff coefficients

  • Description: Standard runoff coefficient for each type of land use.
  • Input: Default value with an option for the user to enter a different value. If the user changes the default, an alert displays in Column K informing the user that the default has changed.
  • Source of Information: MPCA input values based on literature. The default is set at the median. For a discussion of runoff coefficients, see [2]. For a table of runoff coefficients for different soils and slopes, link here.

Columns G and H, Rows 6 through 24 - Pollutant loads

  • Description: The calculated load of the various pollutants of concern. Note that the values contained in Row 24 are the total values for all the land uses. Loading values take into consideration the concentration of pollutant in the stormwater runoff, drainage area, annual rainfall, and runoff coefficient.
This is the base loading from which reductions are made. The User should therefore not change the inputs once the initial calculations are made because it may affect the calculations for pollutant reductions.
  • Input: None
  • Source of Information: The values in these cells are calculated based on the information entered in the previous cells

Columns I, J, K, rows 8 through 14 and row 17 - If the user changes a default value for emc or runoff coefficient in one of these rows, an alert is displayed.

Column L, Rows 8 through 23 - The user may enter notes. For example, if a default value is changed, the user may explain the rationale for the change.

Section 2: Calculation of adjusted total loads

screen shot estimator
Screen shot of Section 2 - Adjusted loads
File:Adjusted loads 2.png
Screen shot of Section 2 - Adjusted loads with example adjustments. Adjustments include implementation of an impervious disconnection program in the industrial are, enhanced street sweeping in the residential area, and conversion of agricultural land.

This section of the 10 calculation worksheets contains information and calculations for adjustments to the total loads calculated in the previous section (Unadjusted total loads). It comprises Cells A26 through L45.

Adjusted total loads account for changes in event mean concentration or runoff coefficient values used in the section "Unadjusted total loads". Examples of practices or actions that result in changes in EMC or runoff coefficients include but are not limited to the following.

  • Street sweeping. Enhancing an existing street sweeping program, or initiating street sweeping will lower the emc in runoff. Alternatively, reducing street sweeping will increase the emc.
  • Implementation of pollution prevention practices. Practices that reduce TP and/or TSS loads will lower the emc. Note that many of these practices are difficult to quantify and the user should base the decision to modify the emc on monitoring data or a literature review. An example would be a neighborhood leaf pickup program.
  • Land use change. A change in land use may result in a change in emc and/or a change in runoff coefficient
  • Altered land use practices. This potentially includes a wide range of practices that may affect the emc or runoff coefficient. Examples include disconnection of impervious surfaces, reduction in impervious surfaces, and changes in lawn practices (e.g. increased yard waste composting).

The user will adjust appropriate emcs in Cells B29 through C44, and/or adjust the appropriate runoff coefficients in cells F29 through F44. Changing a value in one of these cells (shaded yellow) displays an alert box in Column L warning the user that the default has been changed. The user should provide a description or rationale in the appropriate cell in Column K.

The adjacent figure illustrates three example adjustments. In an industrial area, an impervious disconnection program was implemented. This could consist of, for example, a roof disconnection program where roof runoff is diverted to pervious surfaces. In residential areas, enhanced street sweeping lowered the emc. An enhanced street sweeping program might consist, for example, of more intensive sweeping during fall leaf drop. Finally, agricultural land was developed and the emc was lowered for the developed area. For land use changes it is important to avoid double counting. For example, if the newly developed area incorporates infiltration practices, the effect of these practices should be reflected either in this section or in sections 3 or 4, where BMPs are entered, but not in both sections. Including this in both sections would be double counting.

Caution: The user should provide a description or rationale for changed values in the appropriate cell in Column K

Section 3: Calculations for phosphorus load reductions associated with BMP implementation

Tips for entering structural Best Management Practices
Sections 3 (total phosphorus) and 4 (TSS) address reductions in pollutant loading associated with implementation of structural BMPs. Below are some tips for entering data in these sections.
  • The area treated by a BMP cannot exceed the total acreage within a specific land use
  • Examine the values in rows 66-68 for total phosphorus and rows 91-93 for TSS. Guidance on adjusting these and rationale for the defaults is provided here.
  • If multiple BMPs are used in series, consider adjusting the removal efficiency downward for downstream BMPs. For more information, link here.
screen shot estimator
Screen shot showing the section where reductions in phosphorus loading are calculated in the MPCA Estimator.

In this section of the worksheet the user enters acreages for BMPs implemented within each land use in the sub-watershed. The section comprises Cells A47 through L70. The area treated by a BMP cannot exceed the total acreage within a specific land use (i.e. an error message is returned if the user attempts to enter an acreage greater than the total acres in a land use). If the cumulative acreage for multiple BMPs exceeds the total acreage within a land use, Column L returns a message stating the treated acres exceed the total acres for the land use.

Note: it is possible for treated acres to exceed total acres when multiple BMPs are employed. The warning alerts the user to this situation, in which case the user should ensure the entered values are correct. If treated acres do exceed total acres, the user should consider adjusting removal efficiencies for downstream BMPs that are receiving water treated by upstream BMPs.

Caution: The area treated by a BMP cannot exceed the total acreage within a specific land use
Caution: If the cumulative acreage for multiple BMPs exceeds the total acreage within a land use, Column L returns a message stating the treated acres exceed the total acres for the land use.

Rows 48 through 65

  • Description: The area, in acres, treated by the BMP, for different land uses. Land uses are listed in Column A.

These cells represent the area tributary to a BMP, which may not be the entire area for the type of land use. Not all cells need to be filled in. The User should only enter data in the cells relating to the specific BMPs and the land use classification of the BMPs’ drainage area.

If the User attempts to enter BMP acreages greater than the land use area, an error message is generated in Column L since one BMP cannot treat an area greater than the land use area. If the User enters a total acreage for all BMPs that exceeds the area of the land use, a warning message will be generated. This warning does not prevent the User from proceeding, since the total acreage for multiple BMPs may exceed the total land use acreage. For example, assume runoff from a 1 acre commercial area drains to an underground infiltration BMP. The area may also include a 0.2 acre green roof, resulting in 1.2 acres of BMP for the 1 acre commercial area. Generally, it is unlikely the BMP acreage will exceed the land use area.

  • Input: The area, in acres, tributary to the BMP. Different BMPs are contained in different columns.
    • Column B: Biofiltration (bioretention with underdrain)
    • Column C: Infiltration (BMP with no underdrain)
    • Column D: Filter Strip
    • Column E: Landscaped roof (Green roof)
    • Column F: Permeable pavement with underdrain
    • Column G: Sand filter
    • Column H: Swale
    • Column I: Wet basin
    • Column J: Wetland
    • Column K: Other
    • Column L: Displays an alert if total acres treated exceed the acres for a specific land use
  • Source of Information: User

Row 66

  • Description: The removal efficiencies for listed BMPs
  • Input: None or User input. Removal efficiencies are based on information from the Minnesota Stormwater Manual and the Minimal Impact Design Standards (MIDS) calculator. BMP database sources shown below include compilations of data from multiple studies. The User may input other values based on field studies or published BMP performance data. If the user changes the default value, an alert displays in Column L informing the user that they changed a default. The maximum value is 1.0 and an error will generate if the user attempts to enter a value greater than 1.

With the biofiltration BMP, the User will select the removal efficiency based on the type of media present within the BMP. If the media mix used is C or D, or if another mix is used that has a phosphorus content of 30 mg/kg or less per the Mehlich 3 test, then the User should select the phosphorus removal fraction of 0.44. If another media type is present that has a phosphorus content greater than 30 mg/kg then the User should select a phosphorus removal fraction of 0.

Caution: The Estimator assumes engineered media utilized in a filtration practice will not leach phosphorus. For more information, see Engineered (bioretention) media mixes for stormwater applications.

Typically a manufacturer will supply the pollutant removal data for their device. The International BMP Database, USEPA Verified Technologies, Washington State's TAPE Program, and New Jersey's NJCAT Program have pollutant removal information that can be used to verify manufacturer’s data.

  • Source of Information: MPCA Input, but User can change the value.

Links to additional information

Row 67

  • Description: The fraction of the annual runoff that is treated by the BMP. This is based on the amount of total runoff that enters the BMP.
  • Input: None or User input. If the user changes the default value, an alert displays in Column L informing the user that they changed a default. The maximum value is 1.0 and an error will generate if the user attempts to enter a value greater than 1.
  • Source of Information: MPCA Input, but User can change the value. The default value is set for 1.0 for wet basins and wetlands based on the assumption that all annual runoff passes through these BMPs. The default value is set at 0.9 for the remaining BMPs assuming B soils and the BMPs are designed to treat 1.0 inches of runoff from impervious surfaces. One inch is the most common performance goal, but this value must be changed if other performance goals are used. The user should also consider adjusting this value based on soil type curve numbers. The value would be adjusted upward for coarser soils and for areas with increased impervious cover, assuming bmps are sized to treat runoff from impervious surfaces. The User can calculate annual runoff treated by a BMP by using the MIDS calculator, or can estimate the value from the following data from the Minneapolis-St. Paul International Airport. For more guidance, link here.

Daily precipitation vs annual runoff MSP airport

Row 68

  • Description: The fraction of the treated water that is infiltrated by the BMP. Defaults are 90 percent for all infiltration BMPs, 20 percent for biofiltration practices, 20 percent for permeable pavement with an underdrain, and 0 percent for the remaining bmps. All of the water captured by an infiltration BMP is assumed to infiltrate into the soil underlying the BMP. If an infiltration practice is sized to capture more or less than 90 percent of annual runoff, this value should be adjusted accordingly. The MIDS calculator can be used to estimate volume loss in BMPs with an underdrain. Typical infiltration in a BMP with an underdrain and an underlying D soil will be about 20 percent (0.20); thus the 20 percent value for biofiltration and permeable pavement with an underdrain. The assumption that no water infiltrates in the remaining BMPs is a conservative assumption, since some water always infiltrates through the bottom unless the BMP is lined. If the user changes the default value, an alert displays in Column L.
  • Input: None or User input. If the user changes the default value, an alert displays in Column L informing the user that they changed a default. The maximum value is 1.0 and an error will generate if the user attempts to enter a value greater than 1.
  • Source of Information: MPCA Input, but User can change the value.

Row 69

  • Description: The total pollutant reduced (pounds) from each BMP type. The reduction equals the fraction of total area treated by the BMP within each land use multiplied by the removal efficiency of the BMP, summed across all land uses. The total reduction for all BMPs combined is calculated and shown in Column L.
  • Input: None
  • Source of Information: Auto-calculated

Row 70

  • Description: The percent reduction in total load for the pollutant of concern from each BMP type. The percent reduction is calculated by dividing pounds reduced by total load and multiplying by 100. The total percent reduction for all BMPs combined is calculated and shown in Column L.
  • Input: None
  • Source of Information: Auto-calculated

Section 4: Calculations for TSS load reductions associated with BMP implementation

Tips for entering structural Best Management Practices
Sections 3 (total phosphorus) and 4 (TSS) address reductions in pollutant loading associated with implementation of structural BMPs. Below are some tips for entering data in these sections.
  • The area treated by a BMP cannot exceed the total acreage within a specific land use
  • Examine the values in rows 66-68 for total phosphorus and rows 91-93 for TSS. Guidance on adjusting these and rationale for the defaults is provided here.
  • If multiple BMPs are used in series, consider adjusting the removal efficiency downward for downstream BMPs. For more information, link here.
image estimator
Screen shot showing the section where reductions in TSS loading are calculated in the MPCA Estimator.

In this section of the worksheet the user enters acreages for BMPs implemented within each land use in the sub-watershed. The section comprises Cells A72 through L95. The area treated by a BMP cannot exceed the total acreage within a specific land use (i.e. an error message is returned if the user attempts to enter an acreage greater than the total acres in a land use). If the cumulative acreage for multiple BMPs exceeds the total acreage within a land use, Column L returns a message stating the treated acres exceed the total acres for the land use.

Note: it is possible for treated acres to exceed total acres when multiple BMPs are employed. The warning alerts the user to this situation, in which case the user should ensure the entered values are correct. If treated acres do exceed total acres, the user should consider adjusting removal efficiencies for downstream BMPs that are receiving water treated by upstream BMPs.

Caution: The area treated by a BMP cannot exceed the total acreage within a specific land use
Caution: If the cumulative acreage for multiple BMPs exceeds the total acreage within a land use, Column L returns a message stating the treated acres exceed the total acres for the land use.

Rows 72 through 90

  • Description: The area, in acres, treated by the BMP, for different land uses. Land uses are listed in Column A.

These cells represent the area tributary to a BMP, which may not be the entire area for the type of land use. Not all cells need to be filled in. The User should only enter data in the cells relating to the specific BMPs and the land use classification of the BMPs’ drainage area.

If the User attempts to enter BMP acreages greater than the land use area, an error message is generated since one BMP cannot treat an area greater than the land use area. If the User enters a total acreage for all BMPs that exceeds the area of the land use, a warning message will be generated. This warning does not prevent the User from proceeding, since the total acreage for multiple BMPs may exceed the total land use acreage. For example, assume runoff from a 1 acre commercial area drains to an underground infiltration BMP. The area may also include a 0.2 acre green roof, resulting in 1.2 acres of BMP for the 1 acre commercial area. Generally, it is unlikely the BMP acreage will exceed the land use area.

  • Input: The area, in acres, tributary to the BMP. Different BMPs are contained in different columns.
    • Column B: Biofiltration (bioretention with underdrain)
    • Column C: Infiltration (BMP with no underdrain)
    • Column D: Filter Strip
    • Column E: Landscaped roof (Green roof)
    • Column F: Permeable pavement with underdrain
    • Column G: Sand filter
    • Column H: Swale
    • Column I: Wet basin
    • Column J: Wetland
    • Column K: Other
    • Column L: Displays an alert if total acres treated exceed the acres for a specific land use
  • Source of Information: User

Row 91

  • Description: The removal efficiencies for listed BMPs
  • Input: None or User input. Removal efficiencies are based on information from the Minnesota Stormwater Manual and the Minimal Impact Design Standards (MIDS) calculator. BMP database sources shown below include compilations of data from multiple studies. The User may input other values based on field studies or published BMP performance data. If the user changes the default value, an alert displays in Column L informing the user that they changed a default. The maximum value is 1.0 and an error will generate if the user attempts to enter a value greater than 1.

Typically a manufacturer will supply the pollutant removal data for their device. The International BMP Database, USEPA Verified Technologies, Washington State's TAPE Program, and New Jersey's NJCAT Program have pollutant removal information that can be used to verify manufacturer’s data.

Sources of Information: MPCA Input, but User can change the value.

Links to additional information

Row 92

  • Description: The fraction of the annual runoff that is treated by the BMP. This is based on the amount of total runoff that enters the BMP.
  • Input: None or User input. If the user changes the default value, an alert displays in Column L informing the user that they changed a default. The maximum value is 1.0 and an error will generate if the user attempts to enter a value greater than 1.
  • Source of Information: MPCA Input, but User can change the value. The default value is set for 1.0 for wet basins and wetlands based on the assumption that all annual runoff passes through these BMPs. The default value is set at 0.9 for the remaining BMPs assuming B soils and the BMPs are designed to treat 1.0 inches of runoff from impervious surfaces. One inch is the most common performance goal, but this value must be changed if other performance goals are used. The user should also consider adjusting this value based on soil type curve numbers. The value would be adjusted upward for coarser soils and for areas with increased impervious cover, assuming bmps are sized to treat runoff from impervious surfaces. The User can calculate annual runoff treated by a BMP by using the MIDS calculator, or can estimate the value from the following data from the Minneapolis-St. Paul International Airport.

Daily precipitation vs annual runoff MSP airport

Row 93

  • Description: The fraction of the treated water that is infiltrated by the BMP. Defaults are 90 percent for all infiltration BMPs, 20 percent for biofiltration practices, 20 percent for permeable pavement with an underdrain, and 0 percent for the remaining bmps. All of the water captured by an infiltration BMP is assumed to infiltrate into the soil underlying the BMP. If an infiltration practice is sized to capture more or less than 90 percent of annual runoff, this value should be adjusted accordingly. The MIDS calculator can be used to estimate volume loss in BMPs with an underdrain. Typical infiltration in a BMP with an underdrain and an underlying D soil will be about 20 percent (0.20); thus the 20 percent value for biofiltration and permeable pavement with an underdrain. The assumption that no water infiltrates in the remaining BMPs is a conservative assumption, since some water always infiltrates through the bottom unless the BMP is lined. If the user changes the default value, an alert displays in Column L.
  • Input: None or User input. If the user changes the default value, an alert displays in Column L informing the user that they changed a default. The maximum value is 1.0 and an error will generate if the user attempts to enter a value greater than 1.
  • Source of Information: MPCA Input, but User can change the value.

Row 94

  • Description: The total pollutant reduced (pounds) from each BMP type. The reduction equals the fraction of total area treated by the BMP within each land use multiplied by the removal efficiency of the BMP, summed across all land uses. The total reduction for all BMPs combined is calculated and shown in Column L.
  • Input: None.
  • Source of Information: Auto-calculated

Row 95

  • Description: The percent reduction in total load for the pollutant of concern from each BMP type. The percent reduction is calculated by dividing pounds reduced by total load and multiplying by 100. The total percent reduction for all BMPs combined is calculated and shown in Column L.
  • Input: None
  • Source of Information: Auto-calculated

Section 5: Default values for BMP and land use inputs

File:File:Section 5.png
Screen shot showing default values for the estimator.

This section of each of the 10 calculation worksheets contains default values for BMP performance (treatment fractions), event mean concentrations (EMCs), and runoff coefficients. The values for fraction treated are only for the water that is filtered and are expressed as fractions, which represents the fraction of pollutant that is removed by the BMP. A value of 0.50, for example, means the BMP removes half of the pollutant. For infiltration, the fraction represents the fraction of water that is infiltrated. All pollutant is considered removed for infiltration practices. The equations built in to the Estimator account for this complete removal for infiltrated water. Removal values are shown for TP and TSS.

Users cannot change the values in this section. The values are used as a check. When a user changes a default in Sections 1, 2, 3, or 4, comparison of the changed value with the defaults in Section 5 trgger a warning that the User changed a default value.

This section provides additional information and links to information on emcs, runoff coefficients, pollutant removal, and treatment fractions.

Example calculations using the MPCA Estimator

This example utilizes a single site and proceeds through a series of implemented practices.

Step 1. Entering data for land uses

  • MIDS Calculator screen shots for inputs for bioretention without an underdrain (bioinfiltration). Click on an image for enlarged view.
  • Screen shot Estimator example

    Screen shot showing inputs for land uses in Watershed A

  • Screen shot Estimator example

    Screen shot showing inputs for land uses in Watershed B

  • Screen shot Estimator example

    Screen shot showing inputs for land uses in Watershed C



Tips for using the Estimator

The Estimator is a simple tool based on simple assumptions. This section contains some tips for applying the tool to different situations that you may encounter.

Caution: It is Highly Recommended you keep all information you use to calculate reductions

Addressing treatment trains

The Estimator does not explicitly address stormwater treatment trains. It will therefore either underestimate or overestimate pollutant removal for BMPs that are in series, depending on how they are addressed in the spreadsheet. For example, in a treatment train consisting solely of ponds, the first pond removes the greatest fraction of pollutant concentration. Each pond in succession will treat less polluted water but will further the reduction in pollutant concentration flowing downstream. The Estimator doesn’t recognize the total number of BMPs on the ground so it cannot recognize BMPs as being connected. Therefore each pond is assumed to treat the same degree of polluted water regardless of how many are connected in a treatment train.

We recommend that BMPs in a treatment train be considered as possible separate sub-watersheds. Using this approach, only the area draining to a single BMP is considered in a worksheet.

If BMPs in a treatment train are not treated separately, adjusting the Estimator to more closely simulate pollutant removal for treatment trains can be challenging since the Estimator uses a lumped BMP approach in which all similar BMPs are lumped as a single BMP. For example, permeable pavement with no underdrain, bioinfiltration, and infiltration basins are all lumped together as infiltrator BMPs. Another complication is that each treatment train differs and attempting to model them as a single system creates inaccuracies.


Adjusting EMCs (event mean concentrations)

If you are calculating reductions in loading as a percent, there is no value in modifying the EMCs in the Estimator since the only factors affecting the percent removal are the BMP removal fraction and the fraction of runoff being treated by and/or infiltrating through the BMP. If you are calculating reductions in pounds or number of bacteria, the EMC affects the initial pollutant load. The higher the initial load the greater the reduction when the BMPs are applied. For more information on EMCs, go to the following links.

Adjusting the fraction of annual water treated and infiltrated in a BMP

The default value for the fraction of runoff treated by BMPs is 0.9, except for wet basins (e.g. constructed ponds) and wetlands, where the value is 1.0.

  • The 0.9 corresponds with treatment of the first 1 inch of runoff on B soils. This was determined through modeling with the Minimal Impact Design Standards (MIDS) Calculator. If you have a different performance goal (e.g. the 1.1 inch MIDS value), extensive pervious acreages, and/or different soils, we recommend adjusting this value. You can use the MIDS calculator to determine the appropriate value. We also provide some guidance below.
  • The value of 1.0 for wet basins and wetlands assumes all runoff is treated by the BMP. This value should be changed if this condition is not accurate.

The table below can be used to determine the appropriate number. For example, if your soils were A rather than B, you should enter a value ranging from 0.92 to 0.96, depending on the specific soil type. If you had B soils but the water quality volume was 0.75 inches, the value should be changed to 0.81.

Similarly, the fraction of runoff that is infiltrated into an infiltrator BMP is 0.9. Again, this value should be adjusted if the water quality volume or soils differ from 1 inch and B soils or if there are significant pervious acreages contributing to runoff. The only other BMP in the Estimator that infiltrates water as the default is biofiltration. The infiltration fraction for this BMP is 0.2, which is based on data generated from MIDS calculator runs. Infiltration may occur in other BMPs, in particular permeable pavement with an underdrain and swales. A value of 0.2 can be entered for permeable pavement with underdrains to make it similar to biofiltration. An infiltration value for swales is difficult to generate because of the many potential swale configurations. The MIDS calculator is one tool that can be used to generate a value for fraction of water infiltrated in swales.

Annual volume, expressed as a percent of annual runoff, treated by a BMP as a function of soil and Water Quality Volume. See footnote1 for how these were determined.
Link to this table

Soil Water quality volume (VWQ) (inches)
0.5 0.75 1.00 1.25 1.50
A (GW) 84 92 96 98 99
A (SP) 75 86 92 95 97
B (SM) 68 81 89 93 95
B (MH) 65 78 86 91 94
C 63 76 85 90 93

1Values were determined using the MIDS calculator. BMPs were sized to exactly meet the water quality volume for a 2 acre site with 1 acre of impervious, 1 acre of forested land, and annual rainfall of 31.9 inches.


Adjusting the pollutant removal fraction

The pollutant removal fraction is the most important factor affecting the calculations in the Estimator. The default values correspond with recommended values in the manual and represent well-defended data from the literature. The user can change the default, but any change should be supported with data. For further information on pollutant removal by BMPs, see the appropriate page in this manual on BMP pollutant credits. Each of the credit articles contains information to help determine the most appropriate value for pollutant removal for a specific BMP.

Addressing BMPs not included in the Estimator

The Estimator allows the user to enter an additional BMP beyond the default BMPs (called Other in the Estimator). Most urban BMPs fit into one of the default BMPs in the Estimator. There may be exceptions however, including but not limited to the following.

  • Underground filtration practices (these are largely proprietary)
  • Underground settling practices (these are largely proprietary)
  • Floating treatment wetlands
  • Enhanced pretreatment

If you can determine values for pollutant removal fraction, fraction of water that is treated, and fraction of water that is infiltrated for the BMP, you can include an additional BMP. If these values cannot be generated for the BMP, calculate pollutant removal independently and add that value to the value generated by the Estimator when reporting cumulative reductions on the Annual Report form. For example, assume you had an in-line treatment system that treated stormwater runoff in a part of your conveyance system. This BMP is not easily incorporated into the Estimator, but if you monitor the BMP and have pollutant removal information, you can simply add the removal amount to the amount calculated by the Estimator for the remainder of your system.

Adjusting for impervious and pervious surface

The Estimator uses runoff coefficients to estimate the fraction of rainfall that runs off for different land uses. Default values are typical values from the literature. Runoff coefficients can be changed in the Input values for MPCA Estimator tab. Increase the runoff coefficient if your land use has greater impervious surface, or decrease the coefficient if it has less impervious surface. Ranges of values for runoff coefficients can be found here.

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